The present invention relates to hardening of cylindrical hollows such containers, pipes, tubing or the like, made of steel under utilization of a coolant bath, preferably a water bath, the hardening being carried out broadly in conjunction with and following a heat treatment. Generally, the hollow is deemed to have been heated, and is dipped into the coolant (quenching bath) in an orientation wherein the longitudinal axis runs parallel to the surface of the bath of the coolant. Only a portion of the hollow and its surface dips into the coolant at a time. The hollow is rotated about its axis in order to sequentially expose to the coolant all of the material of which the hollow is made.
A method of the type to which the invention pertains, and in which particularly heat removal is guaranteed as far as the entire container is concerned, is for example, discussed in the Russian journal Metalovedenie i Termicheskaya Obrabotka Metalov, number 9, 1985, pages 7 through 10, the title of paper being, in translation, The Hardening of Bottles in a Water-Air Medium.
Upon practising such a method, the number of revolutions of the container to be cooled is kept constant and adjusted such that a maximum rate of cooling obtains. As soon as the surface temperature drops to a particular pre-determined value which is usually in the vicinity of the beginning of the martensitic transition (for example 350 degrees C.), the container is lifted from the coolant bath, quenching is discontinuous and the container will now cool significantly slower in air taking at least 50 minutes, until room temperature is approached. It appears that the only way to reduce the over-all cooling time is by increasing the exposure time to the water bath and by shortening the cooling in air. That, however, is not an appropriate solution, because it was found that cracks and fissures occur in the material, or are at least most likely to occur, thus drastically increasing the failure rate. Obviously this is not a proper approach.